1. Field of the Invention
The present invention relates to a liquid crystal projection system, and in particular to a liquid crystal projection system that utilizes three reflection type liquid crystal panels for image projection output.
2. Description of Prior Art
Because conventional CRT (Cathode Ray Tube) displays are limited by their size and cost, it is difficult for them to obtain a large display. The emergence of liquid crystal protection technologies makes it possible to obtain compact display while ensuring the same display effect as that of a CRT display.
A liquid crystal projector can be classified into a single-panel type and a three-panel type by the number of liquid crystal panels to be used. A single-panel projector is low in resolution and brightness, but is economic in cost. A three-panel projector has the advantages of high resolution and high brightness, but is expensive in cost. The working principle for a three-panel projector is to separate a white light from a light source into three primary color lights of red (R), green (G) and blue (B), then guide the three primary color lights to enter into respective red-color, green-color and blue-color liquid crystal display panels, recombine the three primary color lights reflected from the respective liquid crystal display panels, and finally project them onto a screen via a projection lens.
FIG. 1 illustrates the configuration of a conventional three-panel liquid crystal projection system that is disclosed in U.S. Pat. No. 6,454,416. Such a conventional liquid crystal projection system includes a separating mirror 11, a color selective polarizer 12, a first PBS (Polarization Beam Splitter) 15, a second PBS 17 and a dichroic prism 18. The separating mirror 11 reflects a first primary-color light Bs and transmits second and third primary color lights Rs, Gs from a light source 10. The color-selective polarizer 12 transmits the second primary-color light Rs from the separating mirror 11 with its polarization rotated by 90° to become Rp, and directly transmits the third primary-color light Gs. The first PBS 15 transmits the second primary-color light Rp from the color selective polarizer 12 to a first liquid crystal panel 13, and reflects the third primary-color light Gs from the color selective polarizer 12 to a second liquid crystal panel 14. The second PBS 17 reflects the first primary color light Bs from the separating mirror 11 to a third liquid crystal panel 16. The dichroic prism 18 combines the first, second and third primary-color lights reflected from the respective liquid crystal panels 13, 14, 16 for projection onto a screen via a projection lens. A color selective polarizer 19 is further provided between the first PBS 15 and the dichroic prism 18, and a wave plate or a glass plate 20 is further provided between the second PBS 17 and the dichroic prism 18. Both the color selective polarizer 19 and the wave plate or glass plate 20 are adapted to rotate the polarization of a corresponding incident primary-color light by 90°. Therefore, the polarizations of the three primary color lights that finally enter the dichroic prism 18 become Gp, Rp and Bs. In order to reduce reflection in the light path, the color selective polarizer 12, the first PBS 15, the color selective polarizer 19, the dichroic prism 18, the wave plate or glass plate 20 and the second PBS 17 are cemented together as a unit.
However, in the above conventional three-panel liquid crystal projection system, additional optical elements, i.e., the color selective polarizer 19 and the wave plate or glass plate 20, are further required to be disposed between the dichroic prism 18 and the PBS 15 or 17 so as to achieve the light polarization and separation functions. This decreases the cementing reliability of the prisms 15, 17, 18 and thus results in poor yield and high cost. It is well known that, in a liquid crystal projection system, in order to make the images formed on the three liquid crystal display panels to correctly overlap with each other, each component prism must be made reliable, which mostly depends on the cementing technology for prism. Therefore, any defects in the prism assembly, whether caused by the assembly method or the reliability of an individual prism, will bring deterioration to the entire system and thus adversely affect the image quality.
In addition, the increase of system cementing interfaces will decrease the display contrast and hence adversely affect the image quality. This is because the light absorption of the prisms during work will result in temperature rise, whereby stress occurs at the cementing interface and thus the display contrast is decreased.
FIG. 2 illustrates the configuration of another conventional three-panel liquid crystal projection system that is disclosed in U.S. Pat. No. 7,002,752. This conventional liquid crystal projection system includes three PBSs 21, 22, 23. In order to change the polarization of the incident primary color lights, a color selective polarizer 24 is further provided between the two opposite PBSs 21, 22, and a wave plate 25 is further provided between the two opposite PBSs 22, 23. Consequently, the cementing reliability of the PBSs 21, 22, 23 is also decreased, which correspondingly decreases the contrast of the projection image. In addition, a plurality of polarizers 26, 27 is further provided on one side of the PBS 21 or 23. This increases the number of system components, complicates the system assembly and increases the cost.
The configuration of a further conventional three-panel liquid crystal projection system, which is disclosed in U.S. Pat. No. 6,819,497, is illustrated in FIG. 3. This conventional liquid crystal projection system includes four PBSs 31, 32, 33, 34, which increases the number of system components, the system volume and the cost. Specifically, the PBSs 32, 33, 34 are cemented together as a unit. However, a glass plate 35 is further disposed between the PBSs 32, 34, and a color selective polarizer 38 is further disposed between the PBSs 33, 34. This also decreases the cementing reliability of the PBSs 32, 33, 34.
Further, in the above conventional projection system, the three primary color lights Rs, Gs, Bs emitted from a light source 36 are transformed into Rs, Gp, Bs after passing through a color selective polarizer 37 and then incident into the first PBS 31. Two primary color lights Rs, Bs are reflected by the first PBS 31, transformed into Rp, Bs by a color selective polarizer 30, and incident into the third PBS 33. The primary color light Gp is transmitted through the first PBS 31 and then incident into the second PBS 32. The color light Rp is transmitted through the third PBS 33, incident into a reflective liquid crystal panel 39R, and returns to the third PBS 33 as an S-polarization light Rs after transformation and modulation by the reflective liquid crystal panel 39R. The color light Rs is then reflected by the third PBS 33, transformed into a P-polarization light Rp by a color selective polarizer 38, and incident into the fourth PBS 34. The color light Bs is reflected into a reflective liquid crystal panel 39B by the third PBS 33, and then returns to the third PBS 33 as a P-polarization light Bp after transformation and modulation by the reflective liquid crystal panel 39B. The color light Bp is then transmitted through the third PBS 33 and incident into the fourth PBS 34. The color light Gp is transmitted through the second PBS 32, is incident into a reflective liquid crystal panel 39G, then returns to the second PBS 32 as an S-polarization light Gs after transformation and modulation by the reflective liquid crystal panel 390, and is finally incident into the fourth PBS 34 by reflection of the second PBS 32. Therefore, the primary color lights that are incident into the second, third and fourth PBSs 32, 33, 34 are Gp; Rp, Bs; and Rp, Bp, Gs, respectively. However, it is known that the utility efficiency of the S polarization light in a PBS is 99 percent, while the P polarization light is only 90 percent. The remaining unutilized 10 percent will cause the color phase shift problem due to light interference. Therefore, in the above conventional projection system, since the majority of primary color lights enter the second, third and fourth PBSs 32, 33, 34 in a P-polarization state rather than an S-polarization state, the utility efficiency of the primary color lights in this system is decreased and thus the color phase shift problem may occur.
Accordingly, an improved liquid crystal projection system is desired to overcome the problems as described above in connection with the prior art.